Question:

What is the reason of that apt-add-repository adds PPAs to sources.list.d directory, but repositories links to sources.list file?

Solution:1

AFAIK, (one could check APT's history), the original way was to let PPAs get added to /etc/apt/sources.list. But, as time went on, and the number of possible additional PPAs expanded beyond expectation, it became clear that modifying /etc/apt/sources.list wasn't the best choice. The /etc/apt/sources.list.d/ method was added (maintaining backward compatibility by looking at /etc/apt/sources.list, too). Many packages do it (look at echo /etc/*.d /etc/*/*.d, read man run-parts, although it doesn't apply to PPAs)

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